The primary concern with piping two hydronic systems together is
water from one system mixing with that of the other. Occasionally, there
are two incompatible water treatment regimens used in the two systems
(such as glycol in one system and not the other) which might preclude
any mixing, and thus changeover piping is not possible without adding a
heat exchanger. But more commonly the concern is sending hot water (HW)
into the chilled water (CHW) system and vice versa, thus negatively
impacting temperature control of adjacent coils and increasing load and
energy use at the HW and CHW plants. Three changeover control designs
are shown in Figures 1 to 3 and discussed below. All three designs
include features to minimize the chance of crossflow between the HW and
CHW systems.

Figure 1 includes two 2-way modulating control valves on the HW and
CHW return connections and a 3-way * two-position valve on the supply
connections. Valve actuators are spring return or electronic fail-safe
type with their "normal" position shown as NC (normally
closed) or NO (normally open), defined here as the position of the valve
when there is no power to the actuator. Non-fail-safe actuators could be
used with some additional wiring changes to power the valves to the
desired position, but fail-safe actuators make the design simpler and
more resilient, e.g., if power to the control panel feeding the valves
failed, the valves would fail to positions that ensure HW and CHW
systems are not interconnected. The 2-way valves would usually have
different flow coefficients ([[C.bar].sub.[v.bar]][[K.sub.v]])
corresponding to the design flow rate and full-open pressure drop for HW
and CHW needed to provide good valve authority in each mode. Since only
one valve actuator is active (enabled with control power) at a time, a
single analog output (AO) from the control system can be used to control
both valves. Prior to changing over from one mode to the other, both
2-way valves would be shut, allowing the coil/radiant system temperature
to equalize with the air temperature to reduce the load and energy
impact of mixing HW and CHW and vice versa during the changeover. In
many applications where there is a dead band between heating and cooling
setpoints, this equalization occurs naturally. Changeover is then
initiated using the changeover digital output (DO) from the control
system and relay R-1. This causes the 3-way valve to switch from HW to
CHW (or vice versa), provides power to (thus enabling) the 2-way valve
for the associated mode, and disables power to (thus closing) the 2-way
valve for the opposite mode. In any mode, including in a power outage,
the 3-way valve is open to one of the two hydronic systems to ensure
water from the coil/ radiant system can expand and contract as
temperature changes without creating excessive pressures.

The design in Figure 2 is the same as that in Figure 1 except the
3-way valve is replaced by two 2-way valves with full-closed actuator
end-switches (contact closes when the valve is fully closed). Relay R-2
is provided to allow the HW supply two-position valve to be normally
open, ensuring that water in the coil/radiant system can expand and
contract with temperature changes in case of a loss of power to the
controls. This design costs more due to the extra valve, end-switches,
added relay, and associated wiring, but it solves a potential problem
with the design in Figure 1: as the 3-way valve changes position, a
process that typically takes between 20 and 75 seconds, the HW and CHW
systems are open to each other. Even when the systems have similar
operating pressures (e.g., where elevations and expansion tank locations
and precharge pressures are similar), there will inevitably be a
pressure difference between the two at the 3-way valve, so crossflow
will occur until the expansion tank pressure on the lower pressure
system rises to equalize the two system pressures at the 3-way valve.
The volume of water transferred from one system to the other will be
small where elevations and expansion tank locations and precharge
pressures are similar, a prerequisite when using the Figure 1 design,
but still will result in some inefficiency as HW and CHW mix. The design
in Figure 2 solves this problem by essentially sequencing the valves
using the full-closed actuator end-switches to ensure one valve is fully
closed before the other is allowed to open.

Figure 3 shows a relatively new product offering from a few
manufacturers: a 6-way valve. The typical sequence is as follows: when
the signal from the AO modulates from 2 to 5 Vdc, CHW flow is modulated
from full open to zero flow; from 5 to 7 Vdc, all ports are closed (dead
band); from 7 to 10 Vdc, HW flow is modulated from zero flow to full
open. This design provides all of the benefits of that shown in Figure 2
at a lower first cost and with less chance of failure due to simpler
controls and fewer moving parts. In the control dead band, all ports are
closed, which could cause pressure issues due to thermal expansion of
water in the coil/radiant system, but the valves include a pressure
relief on one port to relieve excess pressure. The only major
disadvantage of this option is that there is a limited range of valve
flow coefficients ([C.sub.v] [[K.sub.v]]) available, so it is not always
possible to find a valve that has the desired flow coefficient for both
HW and CHW; one or the other is oversized or undersized. But it can be
an ideal solution for small coils, radiant panels, chilled beams, etc.

Changeover Coils

Using changeover piping is common for radiant floors, radiant
ceilings, chilled beams, and the like. But it is not common to use
changeover controls in air handlers and fan-coils with a single coil
serving as both the unit's heat/preheat and cooling coil. As with
most design options, there are both advantages and disadvantages. The
discussion below assumes coils are all 8-row (or the maximum available,
such as 6 rows in fan-coils) as recommended in my December 2011 article.
(2)

Advantages of using a single coil with changeover controls include:

* Less space is required in the air handler since there is only one
coil. This was the main driver for using changeover coils for the unique
underfloor air handlers described in my March 2016 column. (3) Because
of the constraint of fitting into a 2 ft (0.6 m) floor module, the air
handlers were too small to fit two coils.

* Extremely high temperature difference ([DELTA]T) can be achieved
on the HW side. On a recent project, design [DELTA]Ts ranged from
90[degrees]F (50[degrees]C) to 110[degrees]F (60[degrees]C) at
160[degrees]F (70[degrees]C) HW supply temperature. This reduces flow
rates by about 60%, making HW pumps and piping smaller, and it improves
condensing boiler efficiency by about 5% due to lower HW return
temperatures.

* For small fan-coils serving small residential and hotel
applications, a single 6-row coil can be used to achieve high [DELTA]Ts
on both the HW and CHW side. Most small fan-coil manufacturers limit the
total number of coil rows to 6, so if separate coils are used, the CHW
coil would be 4-row and HW coil would be 2-row, resulting in low
[DELTA]T on both HW and CHW sides.

* First costs are lower. Added controls costs are more than offset
by the savings from the eliminated coil plus the significant reduction
in HW distribution system costs due to lower HW flow rates.

* Energy costs are lower due to lower pump energy costs, improved
condensing boiler efficiency, and a small fan energy savings due to the
eliminated HW coil pressure drop.

Disadvantages include:

* Extreme stratification will likely occur in heating mode. The HW
is cooled within the first few inches from the coil header because the
coil is so "oversized" for the load. Using a draw-through coil
location will improve mixing but stratification can persist even through
the fan. So if the AHU has a duct split close to the discharge, comfort
problems may result and this design should not be used. The supply air
temperature sensor used to control the coil must be located well
downstream of the coil and an averaging type sensor might be required.

* Because the coil is so effectively oversized in heating mode,
"two-positioning" of the modulating control valve is likely,
with resulting fluctuating supply air temperature. The HW flow rate will
be so small, in most cases the controls will not be readily tunable, and
thus will overshoot setpoint, shut off, overshoot, shut off, etc. But
this is similar to 2-position valves very commonly used for hotel and
residential fan-coils, so not usually a comfort issue.

Controls are more complex where the designs in Figure 1 and Figure
2 are used, such as at large air handlers. (The 6-way valve in Figure 3
is arguably simpler than a traditional design using two 2-way valves.)

* There is the risk of crossflow between CHW and HW systems as
discussed above. But all three suggested designs minimize that risk if
controlled as indicated.

* Coil freezing is possible in cold climates, and even likely on
100% outdoor air coils when outdoor air temperature is below freezing,
due to the very low HW flow rates. Changeover coils should not be used
in these applications unless the systems are protected with glycol. But
in that case, both systems would have to have glycol at similar
concentrations; as noted above, changeover systems are precluded when
the water treatment regimens differ.

Conclusions

Changeover controls have been used for many years on radiant
floors, radiant ceilings, etc., that provide both heating and cooling.
Three controls designs have been proposed that provide reasonable
assurance that crossflow will not occur between hot water and chilled
water systems, the primary concern with changeover designs. These
designs can also be extended to air handlers and fan-coils in many
applications, allowing a single coil to provide both heating and
cooling, resulting in lower first costs and energy costs.

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